Kalsilite, nepheline, carnegieite and eucryptite are all crystalline minerals of ideal composition MAlSiO.sub.4, where M is an alkali metal, having a stuffed silica polymorph-related structure in which the aluminium is tetrahedrally coordinated.
Kalsilite has ideal composition KAlSiO.sub.4, while nepheline exists as a solid-solution and has the composition Na.sub.1-x K.sub.x AlSiO.sub.4, where O.ltoreq.x&lt;1. Both of these minerals have crystal structures closely related to that of the silica polymorph, tridymite (see FIG. 1). Carnegieite has ideal composition NaAlSiO.sub.4 and has a crystal structure closely related to that of the silica polymorph, cristobalite (see FIG. 2). Eucryptite has ideal composition LiAlSiO.sub.4 and has a crystal structure closely related to that of the silica polymorph, quartz (see FIG. 3).
Tridymite, cristobalite and quartz all have the composition SiO.sub.2 and consist of a 3-dimensional framework of corner-connected SiO.sub.4 tetrahedra. Kalsilite, nepheline, carnegieite and eucryptite have been described as stuffed derivatives of the tridymite, cristobalite or quartz structures, in that half of the silica cations in the silicate framework in each case are replaced by aluminium cations. Alkali cations, which are required for charge balance (Si.sup.4+ &lt;--&gt;Al.sup.3+ +M.sup.+, M=alkali) occupy the interstices in the respective frameworks (see FIGS. 1-3)--hence the descriptions "stuffed tridymite", "stuffed cristobalite", and "stuffed quartz".
In kalsilite, nepheline, carnegieite andeucryptite, the interstitial cations, M.sup.+, are not exchangeable under normal conditions, that is, in aqueous salt solution at atmospheric pressure up to -100.degree. C. Therefore, kalsilite, nepheline, carnegieite and eucryptite have negligible cation exchange capacity (CEC). Any CEC is associated with the surface of crystals and not the bulk of the structure.
It has been proposed in, for example, Roux, J., 1971, C. R. Acad. Sci., Ser. D 272, 3225-3227 to exchange the interstitial cations of kalsilite and related aluminosilicates by treating the material at high temperature and pressure under hydrothermal conditions.
It has also been proposed by Sobrados & Gregorkiewitz, 1993, Physics and Chemistry of Minerals, 20, 433-441 to achieve similar exchange of cations by treating kalsilite and related materials with molten salts such as MNO.sub.3 or MCl (M=Li, Na, K, Ag).
However, it is widely accepted that aluminosilicates with the stuffed tridymite-type structure have no cation exchange capacity associated with the bulk structure, either in aqueous solution or in organic solvents.
It has been proposed in, for example, Petranovic et al, 1991, Materials Science Monograph, 666, 2229-2236, that it is possible to exchange the interstitial Na.sup.+ cation of carnegieite with Li.sup.+ by treating it with molten LiNO.sub.3. Associated with this ability to exchange cations by treatment with molten salts is the property of ionic conductivity which has been observed for carnegieite and related materials.
It is also expected that, as for kalsilite and nepheline, exchange of the interstitial cations might be induced under aqueous conditions provided that the material were subjected to sufficiently high temperatures and pressures, i.e. under hydrothermal conditions.
However, it is widely accepted that aluminosilicates with the stuffed cristobalite-type structure have no cation exchange capacity associated with the bulk structure, either in aqueous solution or in organic solvents.
It has been proposed in, for example, Berchot et al, 1980, Journal of Solid State Chemistry, 34, 199-205, that while it is not possible to substitute Li.sup.+ in .beta.-eucryptite by treatment using molten salts with bigger cations such as Na.sup.+, K+ or Ag.sup.+, exchange by divalent cations Cu.sup.2+ and Mn.sup.2+ is possible under such conditions.
However, it is widely accepted that aluminosilicates with the stuffed quartz-type structure have no cation exchange capacity associated with the bulk structure, either in aqueous solution or in organic solvents.
Amorphous derivatives of kaolinite and/or halloysite having large surface areas (BET surface area of at least 45 m.sup.2 g.sup.-1) and cation exchange capacity are described in International patent application WO95/00441 and related applications (including in the United States of America), the contents of which are incorporated herein by reference.
In WO95/00441 the amorphous derivatives of kaolinite and/or halloysite (referred to as "KAD") are produced by a process which comprises reacting a kaolin group mineral with an aqueous alkali halide, wherein the mole ratio of alkali metal halide to the kaolin group mineral is from 5 to the saturation concentration of the alkali metal halide. KAD is characterised as a result of this production process by the presence of halide and substantial amounts of structural water, whether as bound water or as hydroxyl, in its composition.